1. Field of the Invention
This disclosure generally relates to control systems for machines including permanent magnet synchronous motors and/or generators, and more particularly to a system, method and article for detecting the position of a rotor in a machine.
2. Description of the Related Art
The “fuel” powering an alternating current machine, such as a field-oriented permanent magnet synchronous motor, is current. This current may be divided into two components, torque current and flux current. Torque current, Iq, may be viewed as that component of the current that generates motive force, or torque. Flux current, Id, may be viewed as that component of the current that generates magnetic flux in the rotor of the machine. In a permanent magnet synchronous machine, Id may be viewed as the flux and reluctance torque current. Torque and flux currents in induction motors are discussed in more detail in U.S. Pat. No. 6,936,991, issued Aug. 30, 2005 and entitled “METHOD AND APPARATUS FOR MOTOR CONTROL,” which is incorporated herein by reference in its entirety.
Alternating current machines, such as permanent magnet motors and/or generators, are often employed in electric and hybrid vehicles, which typically employ batteries and/or fuel cells as a power source and/or energy storage system. Current is transferred between the windings of the machine and the energy storage or supply system through a power subsystem, typically comprising an inverter and/or a rectifier. Power subsystems typically employ power semiconductor devices, such as insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), and/or semiconductor diodes.
Efficient operation of the machine as either a motor or a generator depends, among other things, upon accurate measurements or estimates of the position, speed and rotational direction of the rotor with respect to the stator of the machine. Some machines employ optical or mechanical encoders to detect the position and speed of the rotor. Such encoders are expensive and not always reliable. Other, so called “sensorless” machines, measure various voltages and currents to estimate the position, speed and rotational direction of the rotor with respect to the stator without using encoders. See, for example, U.S. Pat. No. 6,700,342, issued on Mar. 2, 2004 and entitled “METHOD AND APPARATUS FOR HIGH PERFORMANCE PERMANENT MAGNET MOTOR SPEED CONTROL WITH LIMITED POSITION INFORMATION.” Some machines may combine encoder data with other data to generate appropriate feedback signals. Alternating current machines sometimes have difficulty determining the correct position of the magnetic poles of the rotor. The differences in currents between the north and south poles of a rotor can be small and difficult to detect. Thus, for example, an estimated position of the rotor is often off by 180 degrees. At the same time, application of large voltages is often required to detect the polarity of the rotor, which leads to large currents. Accordingly, it can be appreciated that an improved method, apparatus and article for detecting the position of the poles of a rotor with respect to a stator is desirable.